Inkjet printers have been certainly become popular by virtue of low ambient noise and low running cost, and color printers capable of printing on regular paper have been remarkably spreading into markets. However, it is very difficult to satisfy all necessary properties including color reproducibility of images, wear resistance, light resistance, gas resistance, water resistance, drying property of images, character spreading (feathering), spreading at color boundary (color bleed), concentration nonuniformity at solid portion (beading), printing ability on both sides, and ejection stability, thus an ink composition and a recording medium are selected depending on prior properties of individual applications.
Ink compositions, for inkjet recording, are typically based on water to which a colorant and a humectant such as glycerin for preventing clogging are incorporated. The colorant is mainly water soluble dyes capable of clearly coloring.
However, dyes suffer from poor quality non-allowable on regular paper due to inferior light resistance, gas resistance, and water resistance. In order to address this problem, inkjet-exclusive recording paper having an ink absorbing layer has been used and challenged to improve theses resistances, but there still is a problem that the quality is insufficient.
Pigments have been hence employed in recent years from the viewpoint that pigments can provide excellent light resistance, gas resistance, and water resistance. However, the pigments suffer from inferior coloring property compared to dyes. The reason is believed that lights with different wavelengths and phases interfere in the pigments due to optical multiple reflections.
Therefore, there arise such problems as decrease of color saturation in regular paper and decrease of gloss level in inkjet recording paper when the pigments are employed for a colorant of an ink composition for inkjet recording. In order to address these problems, the color saturation is tried to increase by way of making finer the pigment particles or the gloss level is tried to increase by way of coating a polymer on pigment particles; however, there still exists a difference from the dyes in terms of coloring property.
In addition, highly water-absorbable paper such as the regular paper and the inkjet-exclusive recording paper is employed as the recording media for inkjet recording; such paper typically exhibits an amount of pure water transferred to the recording media of above 35 ml/m2 at a contact period of 100 ms and above 40 ml/m2 at a contact period of 400 ms by measurement of a dynamic scanning absorptometer.
The regular paper having such high water absorbability is advantageous because of being thin and inexpensive, however, suffers from images with poor coloring property and gloom impression since coloring ingredients of ink compositions easily infiltrate into the paper and thus the amount of coloring ingredients on the surface comes to small. The absorbability of the inkjet-exclusive recording paper can be afforded by way of providing a layer of inorganic pigment or water-absorbable polymer on the surface, thus coloring ingredients can remain around the surface and highly coloring property can be induced; however, the inkjet-exclusive recording paper suffers from larger thickness and higher cost beyond comparison with that of the regular paper.
Furthermore, polymer films with no receiving layer on their surface may be exemplified as the recording media. The polymer films typically exhibit an amount of pure water transferred to the recording media of below 2 ml/m2 at a contact period of 100 ms and below 3 ml/m2 at a contact period of 400 ms by measurement of the dynamic scanning absorptometer.
The polymer films as the recording media can scarcely absorb water, thus there often arise such problems as offset smear and ink-void due to spurs (hereinafter sometimes referred to as “spur stain”), beading, and backside smear while stacking, thus the polymer films are far from practical use.
Furthermore, surface-coated paper may be exemplified as the recording media. The surface-coated paper typically exhibits an amount of pure water transferred to the recording media of 2 to 35 ml/m2 at a contact period of 100 ms and 3 to 40 ml/m2 at a contact period of 400 ms by measurement of the dynamic scanning absorptometer. The surface-coated paper as the recording media, which being mainly used for such applications as offset printing and gravure printing, has a less thickness and is not so different with regular paper in terms of price. When images are formed on the recording media using aqueous inkjet inks of which colorant is a pigment, high quality images can be obtained with excellent coloring property and high glossiness since the colorant remains on the surface. However, when the colorant is an aqueous dye, there arises such a problem that the resulting color is of low density and dull since the colorant migrates into substrates (cellulose) through pores of coating layers at the surface together with moisture and aqueous solvent without remaining on the surface.
Furthermore, when pigment inks are employed, highly coloring images can be obtained, however, there arise such problems as spur stain, beading, and backside smear while stacking because of slow drying of ink compositions.
A number of processes are proposed in order to solve the drying of the ink compositions. For example, a process to use a strong alkaline ink is proposed (see Patent Literature 1). However, this proposal suffers from such problems as dangerous handling, tendency to decrease significantly fixability of acid-free paper, easy show through, and remarkable difficulty to record both sides.
A recording process is also proposed to use an oil-based ink composition containing a water-insoluble organic solvent as a liquid medium. However, this proposal is limited to industrial applications in spite of excellent fixability since countermeasures for odor and safety as regards the organic solvent are essential, thus is far from the situation to be employed in offices and general homes.
A process to use an ink composition is proposed in which the ink composition contains a large amount of a surfactant or a surfactant with a lower surface tension (see Patent Literature 2). The ink composition of this proposal may exhibit an excellent drying velocity after printing compared to those of conventional ink compositions, however, hands may be stained upon taking promptly by hand the matters printed at recording apparatuses depending on the species of papers, or significant feathering may occur depending on the species of papers.
A recording process is also proposed in which an ink composition, which being a wax-like solid at normal temperature, is heated and ejected to record under a molten condition (see Patent Literature 3). However, this proposal leads to convex portions of printed characters, which arises such problems as the printed characters transfer and/or papers stick together when a number of the printed matters are stacked and left to stand. In addition, a heating unit is necessary to heat the ink composition, which making the apparatus complicated.
A process to promote the drying is also proposed in which recording paper is heated after printing characters (see Patent Literature 4). However, this proposal suffers from larger electricity consumption since heating rollers and warm-air blowers are necessary, which making difficult to be used in offices or general homes.
As described above, colorants on the basis of dyes exhibit excellent coloring property compared to colorants on the basis of pigments, but represent non-allowable quality when combined with regular paper because the light resistance, gas resistance, and water resistance are inferior as described above.
In order to address the light resistance, gas resistance, and water resistance, processes to use a colorant are proposed in which a dye is dissolved or included into a polymer fine particle to form a colorant (see Patent Literatures 5 to 8). The dye is essentially water-insoluble in these proposals, and the water resistance and the gas resistance are equivalent with those of pigments along with maintaining the highly coloring property of dyes. However, these proposals are limited to recording on regular paper or inkjet-exclusive recording paper, and there arise such problems as occurrences of beading and insufficient drying property when being recorded on recording media with a lower absorbability, specifically, on those of which the amount of pure water transferred to the recording media is below 4 ml/m2 at a contact period of 100 ms and below 5 ml/m2 at a contact period of 400 ms.
In order to solve the problem of drying property in cases where colorants on the basis of dyes are applied to recording media with a lower absorbability, a process is also proposed in which the dye is included into polymer emulsion and the viscosity of an aqueous ink composition is raised (see Patent Literature 9). This proposal may be effective for recording paper with higher absorbability since higher viscosity leads to higher solid content of ink compositions as well as more prompt drying, but is insufficient for recording media with lower absorbability.
Accordingly, the development of an ink-media set is currently demanded that can provide high quality images with higher color saturation and adequate color tone not only on regular paper and inkjet-exclusive recording paper but also on recording media with low ink absorbability.
The recording media for inkjet recording exhibit higher ink absorbability similarly as the regular paper and inkjet-exclusive recording paper, and correspond to those of which the amount of pure water transferred to the recording media is in general above 15 ml/m2 at a contact period of 100 ms and above 20 ml/m2 at a contact period of 400 ms by measurement of the dynamic scanning absorptometer. The regular paper with such adequate absorbability is advantageously thin and inexpensive, however, there arise such problems as show through occurs since color ingredients of inks tend to infiltrate into paper and the coloring property is poor and the images induce gloom impression since the amount of coloring ingredients on the surface comes to small. On the other hand, the exclusive recording paper is provided with the absorbability by way of placing a layer of inorganic pigment or water-absorbable polymer on the surface, thus coloring ingredients can remain around the surface and highly coloring property can be induced; however, the inkjet-exclusive recording paper suffers from larger thickness and higher cost beyond comparison with that of the regular paper.
On the contrary, as regards the recording media that exhibit little ink absorbability such that the amount of ink composition transferred to the recording media is below 4 ml/m2 at a contact period of 100 ms and below 7 ml/m2 at a contact period of 400 ms by measurement of the dynamic scanning absorptometer, polymer films with no receiving layer on their surface may be exemplified as such recording media. The polymer films as the recording media can scarcely absorb inks, thus there often arise such problems as offset smear and ink-void due to spurs, beading, and backside smear while stacking, thus the polymer films are far from practical use.
As regards the recording media having an intermediate ink absorbability among the recording media described above such that the amount of ink composition transferred to the recording media is 4 to 15 ml/m2 at a contact period of 100 ms and 7 to 20 ml/m2 at a contact period of 400 ms by measurement of the dynamic scanning absorptometer, surface-coated paper is exemplified that is mainly used for such applications as offset printing and gravure printing. These recording media are thin and the price is not so different from that of regular paper. There arise in these recording media such problems as occurrences of spur stain, beading, and backside smear while stacking since the drying of inks is slow in these recording media.
When images are formed on these recording media using aqueous inkjet inks, there arise such problems as heavy bleeding and show through, lower density, and gloom coloring, since the colorant under a dissolved condition like water-soluble dyes migrates into substrates (cellulose) through pores of coating layers at the surface together with moisture and aqueous solvent without remaining on the surface.
When the colorant represents a dispersed condition like pigments, the coloring property may be excellent compared to those of water-soluble dyes since the colorant remains on the surface; however, the poor coloring property cannot be improved inherently to pigments, resulting in images with unsatisfactory coloring property.
On the other hand, when water-insoluble dyes such as oil-soluble dyes and disperse dyes are used in a dispersed condition, clear coloring can often be obtained since the colorant can remain at the surface, thus the water-insoluble dyes are considered to be most adequate for recording media with lower absorbability. However, the water-insoluble dye may dissolve into an organic solvent in inks thus the water-insoluble dye partially dissolves into an ink vehicle, and the dye under a dissolved condition causes coloring of clear color inherent to the dye. Therefore, there arise such problems as occurrences of bleeding and show through, low density, and gloom coloring similarly as water-soluble dyes when the dissolved content of dyes is excessive.
As regards the process to solve the problems in terms of the drying property described above, a recording process is proposed to use an oil-based ink composition containing a water-insoluble organic solvent as a liquid medium; however, this proposal is limited to industrial applications in spite of excellent fixability since countermeasures for odor and safety as regards the organic solvent are essential, thus is far from the situation to be employed in offices and general homes.
In order to improve the light resistance, gas resistance, and water resistance of dyes, Patent Literatures 5 to 8 propose colorants in which a dye is dissolved or included into a polymer fine particle. Patent Literature 10 proposes to further add an inorganic pigment. These proposals make possible to take the water resistance and the gas resistance equivalent with those of pigments, however, their applications are limited to recording on the regular paper and the inkjet-exclusive recording paper, and do not address the problems such as beading and drying property where recording is carried out on recording media with lower absorbability like the present invention.
Patent Literature 11 proposes that a pigment ink or a fine particle ink of colored polymer, where a dye being covered by a polymer, is printed on recording media with lower absorbability such as art paper and printing paper like those of the present invention. However, the invention concerns to an ink where the colorant is substantially insoluble into ink vehicle and dispersed in a particulate condition, and does not refers that the coloring property can be enhanced by way of dissolving partially the colorant like the present invention.
Patent Literature 1: Japanese Patent Application Publication (JP-B) No. 63-2996
Patent Literature 2: JP-B No. 60-34992
Patent Literature 3: JP-A No. 02-18710
Patent Literature 4: JP-A No. 2003-72059
Patent Literature 5: Japanese Patent (JP-B) No. 3821306
Patent Literature 6: JP-B No. 3817813
Patent Literature 7: JP-B No. 3839894
Patent Literature 8: JP-B No. 2977817
Patent Literature 9: JP-A No. 2005-306899
Patent Literature 10: JP-A No. 2006-307165
Patent Literature 11: JP-A No. 2006-206666